Outer-Sphere Redox Couples as Shuttles in Dye-Sensitized Solar Cells. Performance Enhancement Based on Photoelectrode Modification via Atomic Layer Deposition

Atomic layer deposition (ALD) has been used to create conformal TiO2 blocking layers on fluorine-doped tin-oxide substrates in dye-sensitized solar cells (DSSCs), effectively eliminating shunting. ALD has also been used to deposit, in controlled fashion, ultrathin coatings of alumina on nanoparticle-based TiO2 DSSC photoanodes. These modified electrodes enable ferrocenium/ferrocene, an outer-sphere redox couple, to be used as a shuttle. The photovoltaic performance and interfacial charge-transfer dynamics were investigated in DSSCs employing this shuttle. It was found that a single ALD cycle is able to passivate surface states, resulting in a dramatic improvement in photovoltaic performance. Subsequent alumina deposition resulted in exponentially increasing electron lifetimes as a function of alumina layer thickness, indicating that the layers behave as barriers to electron tunneling. The characterization of DSSC photovoltaic performance and interfacial charge-transfer dynamics was extended to cells employing derivatives of ferrocenium and ferrocene featuring more positive redox potentials; these cells yielded larger open-circuit photovoltages.